Suppose a terrorist explodes a ''dirty'' explosive device, a bomb to which is attached radioactive materials, inside a building. lf the 'dirty' material is 2.0 kg block of Strontium -90 (9038 Sr, atomic mass 89.9077, half-life of 28.8 years), the Strontium -90 particles would then be scattered throughout the building in the explosion. If the safe intensity for radioactive levels is less than 10 counts/mm, how long will the building be uninhabitable?